MP3421—5.5A, 600KHZ HIGH EFFICIENCY, SYNCHRONOUS, STEP-UP CONVERTER WITH OUTPUT DISCONNECT IN 2X2MM QFN PACKAGE

MP3421 Rev. 1.0

www.MonolithicPower.com

11

11/13/2014

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2014 MPS. All Rights Reserved.

OPERATION

The MP3421 is a 600kHz, synchronous step-up

converter with true output disconnect offered

in a compact QFN 2X2-14 lead package. It

features fixed-frequency current mode PWM

controls for excellent line and load regulation.

Internal soft-start and loop compensation

simplifies the design process and minimizes

MOSFETs, combined with frequency-stretching

operation, allows the device to maintain high

efficiency over a wide-load current range.

Start-Up

When the IC is enabled and IN voltage exceeds

charge period. During this period, the PMOS

rectifier turns on until the output capacitor is

charged to 1.7V. The PMOS current is limited

current. While the output ramps, the PMOS

current limit increases and ramps to 0.7A at

1.7V output.

This circuit helps to limit the

output current under short-circuit conditions.

Once the output is charged to 1.7V, the linear

charge period elapses and the MP3421 starts

switching in normal closed-loop operation. In

the MP3421 operates in step-down mode with

4A typical peak-current limit. Also it works in

with more than 5.5A current limit (see Table 1).

Table 1: Work Mode during Startup

Linear Charge

Mode

Down Mode

Boost Mode

In down mode (if HS-FET is on), the HS-FET

gate is pulled to VIN and works with high

impedance. In down mode, power loss is high

and regulation is poor. Down mode is designed

to work in startup and SCP conditions; do not

set MP3421 in down mode for normal operation

(unless the thermal temperature and regulation

do not affect system performance).

Once the output voltage exceeds the input

voltage, the MP3421 powers its internal circuits

Soft-Start (SS)

The MP3421 provides soft-start by charging an

internal capacitor with a current source. During

the linear-charge period, the soft-start voltage

rises, following the FB voltage. Once the linear

charge period elapses and the voltage on the

capacitor is charged, the reference voltage

slowly ramps. The reference soft-start time is

typically 2ms from 0V to 0.807V.

The

soft-start

capacitor

is

discharged

completely at output in the event of a

commanded shutdown, thermal shutdown, or

short circuit.

Enable (EN)

Operation is enabled when EN is switched high;

it enters shutdown mode when EN is switched

low. In shutdown mode, the regulator stops

switching and all internal control circuitry is

turned off; the load is isolated from the input.

Power-Save Mode (PSM)

The MP3421 automatically enters power-save

mode (PSM) when the load decreases; it

resumes PWM mode when the load increases.

When the device goes into PSM, it lowers the

switching frequency. This prevents switching

and driver losses and switches to pulse-skip

mode if the load continues to decrease.

Error Amplifier (EA)

The error amplifier (EA) is an internally-

compensated amplifier. The EA compares the

generate an error signal. The output voltage of

the MP3421 is adjusted by an external resistor

programs the output voltage via the FB from

2.5V to 5.5V using the equation:

Set the value of R1 and R2 to achieve low-

quiescent current. R1 values larger than 600k